Many current medical infusion devices, such as those for delivering insulin, usually include a rigid container, such as a medical reservoir for the medicine. The containers are generally syringes or other containers having a plunger for administering the medicine. According to standards for medical electrical equipment, it is generally desired to cause an alert if a fill level of the medicine reservoir falls to a certain level. For a rigid medicine container, the fill level can be determined from the position of the plunger within the container. However, medical infusion devices with rigid containers are often limited to the design of the infusion device and, for a given maximum size of the infusion device, are limited in the amount of medicine they can carry.
These design drawbacks can be overcome by using a flexible medicine reservoir that can better utilize the available space within the infusion device. With a flexible medicine reservoir, such as a bag, it is favorable not to push the medicine out of the container by forcing a plunger forwards, but to pump the medicine out the container via suction pressure. Such a pump may also be utilized as a dosing device. This concept is called downstream dosing. The pump fills its chamber from the medicine reservoir and then empties the chamber by administering the medicine in small increments of the chamber volume. This cycle is then repeated. Within this document, the flexible medicine reservoir is also referenced to as medicine reservoir, flexible reservoir or just reservoir, thereby always meaning the same entity.
With a flexible medicine reservoir, there is generally no plunger whose position could be used for determining the fill level of the medicine reservoir. One possibility for determining the fill level of the medicine reservoir would be to integrate the amount of delivered medicine. But such a solution has the drawback that only the amount of medicine taken from the reservoir can be determined, but not the amount of remaining medicine. This is an issue if the initial fill level of the reservoir can vary. In addition, there can be a difference between the amount of medicine to be delivered and the actual amount delivered, which sums up during integration. This means that the medical infusion device could cause an alert even though the reservoir is not emptied to the level where an alert should be generated, or even worse, the medical infusion device could not cause an alert even though the reservoir is emptied below that level and may even be completely empty.